CRUDE OIL SETTLING VESSEL FAILURE ANALYSIS –A CASE STUDY OF VESSELS IN AN OIL/GAS GATHERING FACILITY
Publication Date : 31/01/2017
An analysis is performed to determine the cause(s) of failure of atmospheric, flat-fixed roof cylindrical vessels of vertical design used for transit storage (settling vessel) of hydrocarbon liquid (crude oil) in an oil and gas bearing facility. The vessel is fabricated with normalized low carbon steel (mild steel) plates designed as per API STD 650 and welded per ASME SECTION IX codes. Less than five (5) years in service, the vessels were noticed to be oozing out crude oil from and around the welded seams. This was less than the expected service life, it was designed for twenty-two and half (22 1 / 2 ) years’ service duration. The vessel characteristics, namely, material allowable stress, plate thickness were analyzed for ability to cope with the duty load imposed by the stored liquid. Further, sections were cut from the unfailed parts of the tanks and were subjected to mechanical tests and results compared with specifications. The failed weldment components were examined visually and metallurgically. The observed failures were due to design and welding errors: for instance, the service load on the vessel internal exceeded the maximum allowable static stress of the material. The welding procedure specification was inadequate for the duty as the mechanical properties obtained were lower than required. This led to weakening of the welded seams. To prevent these failures required choice of material (metal plates) with adequate thickness as well as use of assured welding procedure (including effective inter pass temperatures) during welding to achieve the specified mechanical strength.
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